1. Field of the Invention
The present invention relates to a torque detecting apparatus for monitoring the magnitude of a torque imposed on a torque transmitting shaft such as a rotational drive shaft of an electric motor or a vehicle by utilizing a magnetoelastic effect of a magnetic metal and, more particularly, to a torque detecting apparatus which is less influenced by noise or a disturbance magnetic field.
2. Description of the Related Art
A torque is very effective as a fundamental parameter for controlling or monitoring a rotational drive section of an electric motor, a vehicle, or the like.
In order to accurately detect the magnitude of a torque imposed on a torque transmitting shaft with high reliability, detection must be performed in a non-contact manner with respect to the torque transmitting shaft. In order to meet this requirement, a torque detecting apparatus utilizing a magnetoelastic effect produced in an amorphous magnetic alloy has been proposed (Papers Tec. Meet. Magnetics, IEEJ, MAG-81-72). The principle of this torque detecting apparatus will be described below with reference to FIG. 1.
In FIG. 1, reference numeral 1 denotes a torque transmitting shaft. Annular thin strip 2 formed of an amorphous magnetic alloy is wound around and fixed to torque transmitting shaft 1. Induced magnetic anisotropy Ku0 is given to annular thin strip 2 in a direction inclined at inclination angle .theta. (.theta.=0) from circumferential direction 3. For the sake of descriptive simplicity, assume that 90.degree.&gt;.theta.&gt;45.degree., and saturated magnetostriction constant .lambda.s&gt;0. Note that examples of a magnetic metal constituting annular thin strip 2 include ones exhibiting soft magnetism such as an amorphous magnetic alloy, Permalloy (Fe-Ni alloy), Sendust (Fe-Al-Si alloy), and the like.
Assuming that torque 5 is imposed on torque transmitting shaft 1, surface stress o produced on torque transmitting shaft 1 is transmitted to annular thin strip 2. As a result, tension o is produced in annular thin strip 2 in a +45.degree. direction, and compressive stress -.tau. is produced in a -45.degree. direction. The magnetoelastic effect due to this stress causes an induced magnetic anisotropy Ku1 along the +45.degree. direction with reference to the circumferential direction of annular thin strip 2. Note that the magnitude of Ku1 is represented by Ku1=3.lambda..sigma..
As a result, the total magnetic anisotropy exhibited by annular thin strip 2 is changed to a resultant force of magnetic anisotropy Ku0 given in advance and induced magnetic anisotropy Ku1 caused by the magnetoelastic effect, i.e., to Ku2 shown in FIG. 1. By detecting the change in magnetic anisotropy, the stress produced in annular thin strip 2, i.e., a torque imposed on torque transmitting shaft 1 can be detected.
As a means for detecting the change in magnetic anisotropy in annular thin strip 2, a detection coil is conventionally used. The function of the detection coil is as follows. In general, magnetic permeability .mu. of a magnetic substance is changed according to the magnetic anisotropy of the substance with respect to a direction of magnetic excitation. Therefore, when the magnetic anisotropy of annular thin strip 2 is changed, magnetic flux density B in the annular thin strip is changed in accordance with the relation B=.mu.H. As a result, an electromotive force corresponding to the change in magnetic anisotropy of the annular thin strip is produced in a detection coil (not shown) arranged near annular thin strip 2. The electromotive force can be easily measured by a detection circuit connected to the two terminals of the detection coil. Therefore, the change in magnetic anisotropy in annular thin strip 2 and the magnitude of a torque imposed on torque transmitting shaft 1 can be detected on the basis of a change in voltage across the detection coil terminals. In this manner, a torque detecting apparatus of this type comprises an annular thin strip as a primary sensor, and a detection coil as a secondary sensor.
The torque detecting apparatus described above has the following problems.
The magnetic anisotropy of annular thin strip 2 which is the primary sensor is also changed by the influences of magnetic noise or a disturbance magnetic field present in an environment where the thin strip is arranged. As described above, the torque detecting apparatus shown in FIG. 1 operates under the assumption that the change in magnetic anisotropy of annular thin strip 2 corresponds to the magnitude of the torque imposed on torque transmitting shaft 1. Therefore, the influence of magnetic noise considerably impairs detection precision. The disturbance magnetic field or noise can be produced in various directions due to various factors, for example a DC magnetic field along the axial direction of the torque transmitting shaft, a DC magnetic field along the circumferential direction of the torque transmitting shaft, and the like.
This problem disturbs applications on various electric systems mounted on a vehicle, such as a power steering system, a transmission control system, an engine control system, and the like, which have been increasingly developed in recent years. Note that the cause of the disturbance magnetic field includes a magnet, a motor, an electromagnetic clutch, and the like present nearby. When a current flows through the torque transmitting shaft, the current also causes a disturbance magnetic field along the circumferential direction of the torque transmitting shaft.